In general, a communication system has been developed to provide high quality service capable of transmitting and receiving massive data at high speeds. In particular, demands for wireless communication network environments have recently increased. Due to a limited battery life of a mobile station (hereinafter, referred to as “MS”), power consumption of the MS is one of the factors significantly affecting overall performance of the wireless communication systems.
Sleep mode of a base station (hereinafter, referred to as “ES”) and an MS can be employed to efficiently reduce power consumption of the MS. For example, the IEEE 802.16e standard for communication systems supports sleep mode, which is mandatory for the BS. Further, implementation of sleep mode is optional for the MS according to the IEEE 802.16e standard, however, the MS not supporting sleep mode keeps monitoring the downlink all the time even when it does not receive data from the BS or transmit data to the BS, resulting in high power consumption.
Sleep mode is composed of repetition of a sleep interval and a listening interval. The sleep interval is the time duration when the MS does not receive downlink data for power saving, whereas the listening interval is the time duration when the MS receives instruction for the existence of downlink traffic addressed to the MS. During the sleep interval, the MS may not supply power to some of the physical components and it does not communicate with the BS.
FIG. 1 illustrates the operation for controlling sleep mode in a communication system.
As shown in FIG. 1, an MS 100 transmits a sleep request (MOBSLP-REQ) message to a BS 120 to switch from active mode to sleep mode in step S141. On receiving this MOB_SLP-REQ message, the BS 120 determines whether to approve the switching request of the MS 100 to sleep mode or not, and transmits a sleep response message (MOB_SLP-RSP) depending on the determined result to the MS 100 in step S143. For example, according to the IEEE 802.16e standard, this MOB_SLP-RSP message contains several parameters such as initial-sleep window indicating the length of an initial sleep interval; listening window indicating the length of a listening interval; final-sleep window base indicating a base for a final sleep interval and final-sleep window exponent indicating an exponent for the final sleep interval, which are necessary to determine the maximum length of a sleep interval; and start_frame_number indicating the number of the starting frame of the initial sleep interval. In step S143, the BS 120 may request the MS 100 to start sleep mode by sending a sleep response (MOB_SLP-RSP) message without receiving a sleep request message from the MS 100, which is called an unsolicited manner. Upon reception of the MOB_SLP-RSP message, the MS 100 goes to sleep mode at the beginning frame M of the initial sleep interval, and the sleep mode lasts for the length of the initial sleep interval N1. After the sleep interval, the MS 100 enters a listening interval with a length of L.
During the listening interval, the BS 120 transmits a message instructing the MS 100 to switch to access mode if there is any downlink data destined for the MS 100, whereas the BS 120 transmits a message instructing to remain in sleep mode to the MS 100 if there is no downlink data.
Subsequently, during the listening interval right after the initial sleep interval, the BS 120 transmits a traffic indication (MOB_TRF-IND) message with negative indication for the MS 100 since the BS 120 has decided there is no downlink data for the MS 100 in step S145. This MOB_TRF-IND message with negative indication does not require the identification of the MS 100 and the MS 100 having received this message continues its sleep mode. The length of the next sleep interval of the MS 100 is 2×N1, which is a double of the length of the previous sleep interval. That is, if a MOB_TRF-IND message contains negative indication for the MS 100, the length of the next sleep interval of the MS 100 doubles the length of the previous sleep interval until it reaches up to a maximum length N2 of the sleep interval. After the sleep interval has ended, the MS 100 enters a listening interval with a length of L. For example, the sleep interval and the listening interval as described above are defined by Power Saving Class of type I in the IEEE 802.16e standard.
Thereafter, if provided with a protocol data unit (PDU) for the MS 100, that is, if the BS 120 determines that there is downlink data intended for the MS 100, it transmits a traffic indication (MOB_TRF-IND) message with positive indication for the MS 100 in step S147. This MOB_TRF-IND message with positive indication has the identification of the MS 100. The MS 100 having received this message switches to access mode, thereby receiving the downlink data.
In the wireless communication networks, while the BS and the MS perform normal operations in access mode for data transmission or reception, they may enter sleep mode by minimizing the data transmission or reception in order to save power, thereby reducing power consumption of the MS.
The method for controlling sleep mode described above by referring to FIG. 1 is called the binary exponential algorithm. This algorithm is known to be adequate for a packet-by-packet service, which stores data as a unit of packet in a buffer of the BS and transmits packets to mobile subscribers.
FIG. 2A shows a configuration of sleep mode when the IEEE 802.16e communication system employs the binary exponential algorithm, whereas FIG. 25 shows a configuration of sleep mode when the IEEE 802.16m communication system employs the binary exponential algorithm. In the IEEE 802.16e communication system as shown FIG. 2A, the length of each listening interval L is identical, whereas if there is no downlink data intended for the MS 100, then the length of a sleep interval So is doubled to S1 and S1 is doubled to S2, and so on. In the IEEE 802.16m communication system as shown FIG. 2B, an initial sleep cycle C0 consisting of a sleep interval is extended twice to the next sleep cycle C1 consisting of a listening interval L and a sleep interval S. If there is no downlink data intended for the MS 100, then C1 is extended to C2, and so on. If there is any downlink data intended for the MS 100, then during the listening interval, the data can be delivered to the MS 100.
In recent years, the communication systems have been developed to provide a high data rate service and have operated a scheduler to transmit multiple (bulk) packets or variable length packets to an MS at a time.
However, if the conventional binary exponential algorithm is applied to these latest communication systems, power efficiency achieved by the sleep mode is reduced since transitions between access mode and sleep mode are unnecessarily frequent, it may degrade overall performance of the wireless communication systems.